Differential changes in synaptic inputs to ON & OFF retinal ganglion cells during retinal degeneration

© 2015 Dr. Susmita SahaRetinitis pigmentosa (RP) is a family of inherited retinal degenerations caused by the gradual loss of the light sensitive cells in the retina, known as photoreceptors. Without the ability to transduce light energy into biological signals, RP eventually leads to complete blindness, even though many of the non-light sensitive elements of the retinal nervous system remain electrically active. Prosthetic retinal implants, which convert the output from digital cameras into patterns of electrical stimulation that can activate the retina, have successfully restored some useful vision to blind patients that have RP. However, many challenges remain before retinal prostheses are fully mature, i.e. capable of mimicking normal retinal function. One major issue is to be able to differentially stimulate the ON and OFF retinal ganglion cells (RGCs), which transmit retinal information between the eye and brain. It is important to know the functional conditions of these two types of cells in RP, especially at the advanced stages of the disease. The overall aim of my PhD project was to demonstrate the differential effect of complete photoreceptor loss on the synapse density, synaptic currents and spiking activities of ON and OFF retinal ganglion cells using the rd1 mouse as a model of degeneration. I used immunolabeling, image analysis and whole-cell patch clamp techniques to answer my questions for this study. The immunohistochemical studies in the first part of the project revealed that at the stage of complete photoreceptor loss, ON-RGCs showed a significant reduction in excitatory synapse density while OFF-RGCs showed a significant reduction in the inhibitory synapse density. Distribution patterns of both the excitatory and inhibitory synapses across the dendritic fields of RGCs were unchanged. The change in synaptic input was associated with a reduction in the density of ON-cone bipolar cells. In the second part, patch clamp electrophysiology experiments conducted on the same cell types in the same mouse model at the same stage of degeneration revealed a significant reduction in spontaneous excitatory post-synaptic current (sEPSC) frequencies and a significant upregulation in spontaneous inhibitory post-synaptic current (sIPSC) frequency in ON-RGCs, but not in OFF-RGCs. OFF-RGCs only showed significantly higher amplitude EPSC events. Finally, we found a strong correlation between the results in the first and second part of this study, which suggests that the ON and OFF ganglion cells are differentially affected by complete photoreceptor loss in RP